Introduction to Combustion and Fire - Coursework Example

1. Matter. International Systems of units (SI) Exercises 1. Reduce the following dimension to its simplest form: (Power/Pressure)1/3 Solution: Power = Watts/time = kgm2/s4 Pressure = Newton/meter2 = kg/ms2 (Power/Pressure)1/3 2. Convert in the SI units the following values: a) 5cm/microsecond Solution: b) 0.36 x 10 - 10 tons km/min2 Solution: 2. Chemical Elements and Compounds Theoretical question 1. What are free atoms and radicals? What is the difference between an ion, free atom and radical? Answer Free atoms are atoms whose properties like magnetic moment are not significantly affected by nearby atoms, molecules, or ions. Radicals on the other hand are atoms, ions, or molecules with an unpaired electron (an open shell configuration) which is highly reactive. Radicals differ from ions since radicals do not possess a charge whereas ions carry either negative or positive charge. Exercise 1. Explain chemical bonds in the molecule of methane. Answer Methane CH4 has covalent bonding. Covalent bond is a type of chemical bonding characterized by the sharing of electrons between atoms. It is introduced by Lewis in the Lewis’ electron dot notation. The Carbon in Methane which has 4 electrons covalently bonds with the electrons of Hydrogen through attraction-to-repulsion stability to form methane molecules. 3. States of Matter: Fluids, Solids and Gases Exercise 1. Calculate the vapour densities (kg/m3) of pure C5H12 at 25oC and 1 atm = 1.013 105 Pa. (Assume ideal gas behaviour.) Solution: Atomic Mass of C = 12.01 4. Chemical Reactions and Their Rates Theoretical questions 1. What are the stoichiometric, fuel lean and fuel rich mixtures? Give an example. Answer: Stoichiometric mixture refers to the air-fuel ratio that usually occurs when fuels are combined with free oxygen and the mixture turns out to be chemically balanced. The stoichiometric air-to-fuel ratio is approximately 14.7 to 1. A fuel rich mixture occurs when the air-fuel ratio is below 14.7:1 and a fuel lean mixture occurs when the air-fuel ratio is more than 14.7:1. Spark ignition engines are run by fuel rich mixtures while compression-ignition engines are run by fuel lean mixtures. 2. What is the concentration and its units of measurements? What is a mole? Answer: Concentration is the measure of how much of a given substance is mixed with another substance in a mixture. Concentration is measured in mass, volume, and molarity or molar concentration. A mole is a physical quantity that measures the amount of substance present in a system with elemental quantities. 3. Explain temperature and concentration dependence of the chemical reaction rate and Arrhenius equation. Answer: In collision theory, the molecules of the reacting substances need to have enough energy in order to collide (and for the chemical reaction to occur). Increasing the concentration of the reactants increases the frequency of collision and thus speeds up the rate of chemical reaction. Arrhenius equation also depends on the temperature and on the concentration of reactants. According to the Arrhenius equation, chemical reaction would only occur if enough number of molecules (concentration) will overcome the activation energy (temperature) needed to produce a noticeable reaction. Thus, increasing the concentration of the reactants as well as their temperatures increases the possibility of breaking the activation energy of the reaction. Exercise 1. Compare the chemical reaction rates at two temperatures T1 = 300 K and T2 = 600 K for activation energy E = 180 kJ/mole. The universal gas constant is 8.314 J/(mole K). Solution: This means that the chemical reaction rate given the activation energy Ea = 180kJ is increased by a billion billion million times when temperature T is increased. 5. Thermal Explosion Theoretical questions 1. Analyse thermal explosion in adiabatic conditions and the mechanism of self-accelerating reaction. Define induction period. Answer: In order to understand adiabatic thermal explosions, it is good to start with the adiabatic process. Adiabatic process is a thermal expansion that does not involve any transfer of heat from or to the system. At t=0, a gas of constant volume is instantaneously heated to a temperature T. assuming that the heat loss from all the involved mechanism is negligible compared to the thermal energy of the chemical reaction, the mixture will ignite after a very short time defined by the equation which means that the ignition time decreases with the heat release q, the reactivity of the mixture, the increase in the specific heat, and the mass fraction of the fuel. Self accelerating reaction occurs before a thermal explosion in adiabatic condition happens. When the fuel mixture reach the condition where its temperature in instantaneously increased to T, self accelerating reaction occurs. Induction period is the slow phase of a chemical reaction before accelerated reaction occurs. Exercises 1. Consider qualitatively Semenov diagram for thermal explosion in a vessel with cold walls. Explain the effect of initial temperature and size of the vessel on the critical conditions for thermal explosion to be possible. Answer: Thermal explosion in a vessel with cold walls is dependent on the initial temperature and the size of the vessel. The initial temperature of the vessel is a determining factor for the possibility of the explosion. The lower the initial temperature To is, the longer it takes for the explosion to occur because the rate with which the combustible material releases heat to the volume would take time to increase the heat of the cold walls. . The relationship between the critical condition for thermal explosion and the volume (or size of the vessel) is apparent and can be observed through the equations of heat release rate per unit volume q+ and heat loss per unit volume q. The larger the volume, the longer it takes for the combustible mixture to heat the walls of the cold vessel. Besides, the vessel will have enough area to release the heat it gained from the mix inside its walls, so explosion would have less chance to occur. 2. Calculate the induction period for adiabatic thermal explosion of flammable pyrotechnic mixture (polyvinilnitrate) upon various initial temperatures. Initial temperatures: T00 = 300 K, T0 = 600 K, T0 = 900 K. Solution: 6. Forms of Heat Transfer Exercise 1. Calculate the rate of heat transfer through a 0.4 squire meter of a plaster wall 3 cm thick. One side of the wall is at 600 0C, while other side is at 25 0C. Thermal conductivity of plaster is 0.5 W/m x 0C. Solution: 7. Ignition Theoretical question 1. Describe the process of ignition of a solid combustible material by a hot plate. Explain evolution of the temperature field in the solid material. Answer: Combustions that occur between oxygen and solid surfaces (which serves as fuel) are called smoldering. Smoldering occurs when there is a combustion reaction on solid surfaces (ignition) where the combusting solid undergoes glowing (with a temperature T>1000oC) followed by charring (or the production of carbonaceous residues). A hot plate may ignite a solid combustible material 8. Premixed Flame Theoretical questions 1. Why do flames propagate through a combustible mixture? Answer: Flames propagate through a combustible mixture because of the presence of the elements that would encourage the propagation of flame such as source of fuel, presence of oxygen, and source of ignition. When the combustible mixture is ignited, flames are expected to propagate in the mixture because of the presence of the other two elements that constitutes flame production (i.e., the source of fuel which is the combustible mix and oxygen). If the combustible mixture is a liquid mixture, fire spread is driven by surface tension and temperature. As the temperature increases, surface tension decreases. At the surface of the liquid, the temperature is low at the head of the flame front which propels hot fuel from beneath the flame, spreading the fire. 2. What are the flame front and flame propagation velocity? Why a gas particle entering flame front is accelerated? Answer: Flame front is another name of the combustion zone. It is the area of fire where combustion occurs. Flame propagation velocity is the velocity of the flame relative to the initial air-fuel mixture in the direction normal to the surface of the flame. Gas particles entering the flame front experience acceleration because of the heating and expansion. 3. What is adiabatic flame temperature? Answer: Adiabatic flame temperature is the resultant temperature of a combustion that occurs without any work, heat transfer, or changes in the kinetic and potential energies. Exercise 1. The angle of a premixed flame front cone stabilized on the Bunsen burner is 450. The combustible mixture velocity in the tube is 2 m/s. What is the flame propagation velocity? Answer: 9. Detonation Theoretical questions 1. Compare the main features of premixed flame and detonation. Answer: Premixed flames are flames that are produced when the oxidizing agent is mixed with fuels before the oxidizer reaches the flame front. Premixed flames are characterized by flame speed. Detonation on the other hand is the violent and instantaneous release of chemical, nuclear, and mechanical energy with the propagation of shock waves traveling at supersonic speeds. The main difference between premixed flame and detonation is the speed with which flames are generated. 2. Describe internal structure of detonation wave. Answer: Detonation waves are shocks that are accompanied by exothermic reaction. The propagation of wave velocity in detonation is supersonic and waves traveling under this condition are traveling through a highly combustible or chemically unstable medium. The internal structure of detonation wave looks like the following: The diagram shows how the wave propagates in a medium at a speed D (typically on the 103 scale). The area between -Δ and 0 is the induction zone. When –Δ = 0, the wave is in the chemical reaction zone. The point where x=0 is the shock front. Numerical exercise 1. Calculate the velocity of steady-state freely propagating strong detonation if the ratio of specific heat capacities is 1.26 and heat of combustion 750 kJ/kg. Solution: 10. Diffusion combustion. Fire Plume Theoretical questions 1. Compare a jet fire and buoyancy dominated fire, using the flame height-jet velocity diagram to explain flame shape. Answer: Both jet fire and buoyancy dominated fire depends highly on the velocity of fuel injection to determine its final state. Jet flames are flames produced when flow rate of the fuel is high and produces turbulent flows. Buoyancy dominated fires on the other hand have low flow rate of its fuels. This property of jet fires can be seen in the height-jet velocity diagram. 2. Explain the low value of the Froude number for natural fires. Describe fluid-dynamic structure (air entrainment, buoyant flow, eddies) of a fire plume. Answer: Froude number is expressed by the equation where V is the velocity of gases, g is gravitational constant, and D is plume width or fuel dimension. Froude number measures the relative importance of inertia (or momentum) and buoyancy in the system. The numerator denotes momentum while the denominator of the equation speaks about buoyancy forces (or gravity). Natural fires occur when fuel and oxygen (which are initially separate) burn in the region where they meet. Buoyancy is the dominant driving force of natural fires and buoyant flow determines the shape and size of natural fires. It means that the flames coming from natural fires have no structural order and are easily affected by external fluid motions like air movement. Natural fires are characterized by the presence of eddies. Because natural fires depend highly on buoyant forces, with fire momentum almost zero (because of the absence of intimate fuel mixing), the fractional relationship between numerator (momentum/inertia) and denominator (buoyant forces) is low. Typical value of Froude number for natural fires is 1 (high velocity diffusion flames have Froude numbers as large as 7). A buoyant plume is a column of flame and hot products of combustion that rise above the surface of the fuel. As hot air enters the buoyant plume, cold air is introduced in the column which results to an entrainment. Eddy currents are produced when the tip most portion of cold air meets the end portion of rising hot air. 11. Combustible Liquids and Solids Theoretical questions 1. Describe the flash point, fire point and auto-ignition temperature for combustible liquids. How these characteristic are measured in laboratories? Answer: Flashpoint is the lowest temperature T where a mixture of air and flammable vapor exist at any given surface. Fire point is the lowest temperature T where ignition in an open cup is followed by sustained burning. Auto-ignition temperature is the phenomena when the mixture of air and vapor self-ignites. These three characteristics of combustible liquids are measured by slowly heating a combustible fluid, about 5o to 6o per minute, where a small pilot flame is introduced into the vapor space at frequent intervals. The temperature that would start the expected reaction/combustion is the point of interest. 2. What is BLEVE? Explain possible effects of accidental liquid fuel releases on the surrounding. Answer: BLEVEL stands for boiling liquid expanding vapor explosion which refers to the explosion that occurs when a pressurized vessel containing pressurized liquid ruptures. BLEVE are hazardous as it may destroy the vessel containing the pressurized liquid, sending shrapnel and debris that may hurt people and property. 3. Discuss the main factors influencing flame spread over solid materials. Answer: The factors that influence the flame spread over solid materials are the same factors that influence the flame spread over combustible mixtures. First there has to be a source of fuel, a source of ignition, and enough oxygen to sustain fire. Spread of flame in solid materials is aided by other factors as well such as the direction of the wind and the buoyancy force. Two types of wind spread in solid materials are opposed flow flame and wind-aided flame spread. Numerical exercise 1. Calculate the average flame height for a pool gasoline fire. Diameter of pool is 4m. Solution: 12. Fire as a Combustion System Theoretical questions 1. Define heat of combustion, heat release rate and combustion efficiency. Answer: Heat of combustion is the amount of energy that is released when one mol of a compound undergoes complete combustion with oxygen. Heat release rate is the amount of heat released by a pool of fire. Combustion efficiency is the measure of how well the heating equipment converts fuel into useable heat energy. 2. Describe three zones in turbulent diffusion flame (fire plume). Answer: 3. Why is thermal radiation of importance in fire? Answer: Exercises 1. Estimate the gas velocity in the fire plume. Answer: 2. Estimate the Froude number in fire plume. Answer: 3. Calculate the heat release rate for PMMA (m''=0.035 kg/m2 s, heat of combustion 23.0 MJ/kg, combustion efficiency 0.6. Solution: 13. Fire in Enclosures Theoretical questions 1. What is a positive thermal feedback for fires in enclosures? What is flashover and back draft? Answer: 2. Describe the conditions necessary for flashover to occur in terms of radiant heat flux at floor level, temperature of a hot upper layer, and minimum required heat release. Answer: 3. Explain fuel-controlled and oxygen-controlled regimes of fire in an enclosure. Answer: 4. Describe the main flow patterns associated with fire development in enclosures. Answer: 14. Fire in Enclosures. Fire Modelling Theoretical questions 1. What are advantages and limitations of zone models? Answer: Fire zone models are computational analysis employed by fire engineers to analyze the compartment under consideration by dividing the compartments in zones and analyzing each zone separately. Data gathered from separate analysis are pooled to come up with conclusive data of the whole process. The basis for zone models is the conservation of mass and energy of fire in the compartment and analyzes the parameters heat release of the combustion of material, fire plumes, smoke movements, mass flow, and gas temperatures. The major advantages of zone models are its ability to capture an accurate model of the compartment and its ability to easily incorporate the dimensions and openings of a compartment. Its major limitations are its less comprehensive approach to modeling factors that pertains to fire and the mathematical concepts it use to analyze and model fire. 2. What is field modelling of fires? What are objectives of CFD fire modelling? Answer: Quantitative analysis of fire is difficult because of the natural lack of form where fires can be analyzed, as in analyzing motion. Scientists and engineers employ mathematical analysis in order to recreate fires and be able to understand its properties and the extent of destruction it can do. The mathematical model used by scientists and engineers is computational fluid dynamics fire modeling. CFD fire modeling works like FEA in that it represents the enclosure under consideration in grids (or meshes) and applies control volumes, constraints, and the appropriate values to analyze the propagation and the spread of fire. Typical CFD fire modeling software programs are using Navier-Stokes equation for numerical analysis. 3. Describe verification and validation for field modeling? Why validation is necessary? Answer: It is not enough that fire models are presented to explain the propagation and spread of fire. It is also necessary to verify the results of the fire model and validating the results. Verification ensures that the data input are correct and all the factors are taken in for fire modeling. Verification includes checking and rechecking the data inputs for accuracy and precision. For example, exact values of dimensions of the compartment, the air-fuel mixtures, and the speed of fire propagation are being input in the model. Validation on the other hand ensures that the result of the analysis is consistent. Validation involves running the analysis for a number of times. Verification and validation is necessary to make sure that the result of the fire modeling analysis is the best approximate of the event. Read More